A large number of integrated circuit (IC) devices may be formed on a single wafer or substrate of semiconductor material. Each substrate is a thin slice of a single crystal semiconductor material such as silicon. The successful formation of viable IC devices requires the use of a correctly formed and processed substrate. An individual substrate may undergo steps of rough polishing and chemical-mechanical polishing (CMP) to remove surface damage caused by substrate slicing, to achieve a desired thickness, and to produce a substantially flat and planar surface on the substrate. The substrate edges may also be ground to a rounded configuration.
Typically, an initial step in forming IC devices on a silicon substrate is oxidation of the surface, such as to silicon dioxide SiO2, for example. The SiO2 provides a hard, electrically insulating layer which also serves to protect the substrate surface from physical damage and contamination. The formation of IC devices on a substrate normally includes many steps, most of which may be classified in the broad categories of layering, patterning, doping, and heat treatment. Every type of semiconductor chip requires some type of isolation in order for the individual devices (e.g., transistors, capacitors, resistors, etc.) to operate independently of one another, or to operate in environments of high radiation.
Many of the defects which occur in substrate and chip manufacture are related to inadequate dielectric isolation. Because of the drive toward integrated circuit devices with greater density and complexity, individual components must be made increasingly smaller and placed closer together. In some applications, operation at higher voltages or in high radiation environments is required. Thus, the need for electrical isolation and radiation isolation become of much greater significance.
One conventional method of forming a dielectrically isolated (DI) substrate for manufacturing bipolar and metal-oxide-semiconductor (MOS) devices will now be described. A prior art substrate is conventionally formed as a slice of a single crystal silicon material, and typically is subjected to grinding, polishing and surface oxidation steps to form a smooth planar first substrate surface. The first substrate surface is etched in a V-groove etching method well-known in the art. The etched depressions are known as trenches, troughs, or pockets, and may be formed by an isotropic wet etch or an anisotropic dry etch. In this application, the etched depressions will be referred to as trenches.
The side surfaces and bottom surfaces of the trenches and the non-trenched surfaces are oxidized to form a layer of silicon dioxide over the surfaces of the trenches and the non-trenched surfaces. This oxide layer is the isolation barrier. A thick layer of polysilicon is deposited over the oxide layer (i.e. the isolation barrier). The polysilicon layer backfills the trenches and non-trenched portions of the first substrate surface, forming a “handle” or layer for supporting the substrate and devices formed thereon. The substrate is lapped or removed from its second surface until the oxide layer is reached.
A substantial portion of the original silicon is removed, leaving a smoothed polysilicon substrate surface including oxide-isolated tubs or islands of the original single crystal silicon material. The exposed surface of each tub has an active surface. Circuit components may be fabricated on the active surfaces of the silicon tubs. The completed substrate with circuit components thereon may be cut or singulated into discrete chips which are packaged for the intended use.
The manufacture of DI devices has presented several drawbacks. First, unless the etching steps are carried out very carefully, the final working surface of the substrate may not be as planar as the original substrate. This affects subsequent processes, especially lithography, and may produce islands or tubs with varying thicknesses. Another problem in DI device manufacture has been the formation of pinholes and other defects in the isolation barrier (i.e., the oxide layer). Such pinholes cause current leakage to occur in both normal and high radiation environments, effectively negating the purpose of the isolation barrier. Thus, integrated circuits manufactured with the DI method have additional risks of reduced performance and poorer reliability.
Current leakage defects present a major problem in the manufacture of DI substrates and are largely due to the presence of contaminants in or on the surface of the oxide layer which become activated upon application of the polysilicon layer. The unit production cost of DI devices has been relatively high, largely because of the resulting low substrate yield.
An early form of isolating a substrate is described in U.S. Pat. No. 3,571,919 to Gleim et al. The patent discloses depositing a layer of silicon carbide over mesas which, after etching become individual islands of single crystal silicon isolated by the carbide layer. The use of silicon dioxide as an isolation barrier became well-known, as indicated in U.S. Pat. No. 5,114,875 to Baker et al. This reference addresses the formation of metal conductors spanning the isolation barrier.
In U.S. Pat. No. 5,206,182 to Freeman, a trench isolation process is used to form electronic circuits surrounded on lateral sides by air-filled trenches. In intermediate steps of fabrication, the inner and outer walls of the trenches are coated with a silicon dioxide layer and covered with a layer of silicon nitride. The trenches extend downwardly to a level at or below a buried layer.
None of the above references address the problem of pin-hole formation and resulting current leakage in DI substrates when a silicon dioxide isolation barrier is covered with polysilicon. One attempted approach has been to decontaminate the polysilicon deposition chambers more frequently, i.e., between each batch. This has not substantially reduced contamination. Furthermore, such frequent cleaning is time-consuming and expensive.